1. Technical Field of the Invention
This invention relates to temperature detecting devices, and more particularly to a temperature detecting device for detecting a heat on a heater (power transistor or the like) of a power-operated auxiliary steering system mounted on a vehicle, say an automobile, by the use of a heat-sensitive resistance element (element having a resistance value changing sensitive to heat; thermistor, platinum ohmmeter or the like).
2. Description of the Related Art
In the power-operated auxiliary steering system, generally the manipulation amount of a steering wheel is converted into an electric signal and amplified by a power transistor. By an output of the transistor, the motor built in the steering unit is driven to generate an auxiliary steering force commensurate with the manipulation amount.
FIG. 13 is an essential-part device structural view of a power-operated auxiliary steering system. In this figure, 1 is a control unit for converting a manipulation amount of steering wheel into an electric signal, 2 is a power unit, and 3 is a motor. In many cases, the control unit 1 and the power unit 2 are accommodated within the separated cases. This is because of a countermeasure not to cause a malfunction in the constituent part (microcomputer and its peripheral circuits, etc.) of the control unit 1 under the influence of a heat of the power transistor 2a included in the power unit 2.
The power transistor 2a included in the power unit 2 has a heat in an amount increasing with a drive current to the motor 3. It also increases depending upon a temperature of service environment under the blazing sun in the daytime. For this reason, particularly, the power transistor 2a for vehicular application essentially necessitates a preventive measure against thermal degradation. It is a conventional practice to arrange heat-sensitive resistance elements 2b, 2c (reason for providing two elements will be referred later) such as thermistors in locations beside the power transistor 2a. The temperature-detection signal of the same is captured into the control unit 1, to carry out a temperature compensation for reducing the driving current to the motor 3 when the temperature is high.
In the meanwhile, the xe2x80x9cheat-sensitive resistance elementxe2x80x9d such as a thermistor suffers a failure of xe2x80x9cdisconnectionxe2x80x9d or xe2x80x9cshort circuitxe2x80x9d to rarely occur. In the case of a disconnection failure of those, because device output is lost at once, the failure is easy to sense, e.g. measure can be readily taken including to issue an alarm to the driver. However, in the case of a short circuit failure, because symptom gradually proceeds from a partial short circuit into a whole short circuit, device output changes little by little thus making it difficult to sense a short circuit failure. For this reason, in the case of detecting a temperature lower than the actual temperature, the power transistor 2a is driven by an excessive driving current. This results in a disadvantage that the power transistor 2a is deteriorated acceleratedly.
Thus, there is known an art that a plurality of (usually, two) heat-sensitive resistance elements 2b, 2c are provided to compare between the detection signals thereof, making it possible to sense an occurrence of failure (disconnection or short circuit) in any one of the heat-sensitive resistance elements 2b, 2c. For example, Patent Document 1 has two thermistors having the same characteristics (e.g. negative characteristics that resistance value lowers with increasing temperature), so that the end-to-end voltage Va, Vb of the thermistors can be converted into detection temperatures Ta, Tb for extraction. Furthermore, the lower detection temperature of those is employed as a correct detection temperature.
Herein, the reason of xe2x80x9cemploying the lower detection temperature as a correct detection signalxe2x80x9d is because of the following reason. Namely, as described in a passage [0061] of the document, xe2x80x9cin the case that a short circuit failure occurs in any one of the two thermistors, errors are caused in the detection temperature Ta, Tb obtained from the thermistor end-to-end voltage Va, Vb. Because of the negative characteristic of thermistor, the error in the detection temperature Ta, Tb is toward the higher temperature. Accordingly, by employing the xe2x80x9clowerxe2x80x9d detection temperature, the thermistor on which a short circuit failure occurs can be ignored. Eventually, temperature detection is possible without encountering errors.xe2x80x9d
[Patent Document 1]
JP-A-7-190576 (pages 2-7, FIG. 8)
However, the temperature detecting device in the prior art is nothing more than an arrangement with two heat-sensitive resistance elements in locations nearby the heater, or a power transistor, wherein detection signals thereof is captured into the control unit for the purpose of comparison and consideration, thus involving the following problem.
FIG. 14 is an essential-part connection diagram of a control unit 1 and a power unit 2. The power unit 2 has a power transistor 2a as a heater and two heat-sensitive resistance elements 2b, 2c (hereinafter referred to as xe2x80x9ca first heat-sensitive resistance element 2b, a second heat-sensitive resistance element 2cxe2x80x9d) arranged in locations nearby the power transistor 2a. The control unit 1 has two pull-up resistances 1a, 1b (hereinafter referred to as xe2x80x9ca first pull-up transistor 1a, a second pull-up transistor 1bxe2x80x9d) and a microcomputer 1e incorporating (or satisfactorily attached externally with) two AD converters 1c, 1d (hereinafter referred to as xe2x80x9ca first AD converter 1c, a second AD converter 1dxe2x80x9d). Note that xe2x80x9cAD converterxe2x80x9d is an abbreviated form of analog-to-digital converter.
Four lines 3a-3d are laid between the control unit 1 and the power unit 2. The first heat-sensitive resistance element 2b has one end connected to an input terminal of the first AD converter 1c through the line 3a and to a power source VCC through the first pull-up resistance 1a. The other end of the first heat-sensitive resistance element 2b is connected to a ground 1f of the control unit 1 through the line 3b. Meanwhile, the second heat-sensitive resistance element 2c has one end connected to an input terminal of the second AD converter 1d through the line 3c and to the power source VCC through the second pull-up resistance 1b. The other end of the second heat-sensitive resistance element 2c is connected to the ground if of the control unit 1 through the line 3d. 
The disadvantages in this configuration lies in the four lines 3a-3d needed between the control unit 1 and the power unit 2, the two pull-up resistances 1a, 1b needed in the control unit 1, and the two AD converters 1c, 1d needed in the control unit 1. Eventually, the disadvantages are to pose a problem of increasing the number of parts and raising the cost of manufacturing.
Therefore, it is an object of the present invention to provide a temperature detecting device capable of reducing the number of lines between the control unit and the power unit, the number of pull-up resistances within the control unit or the number of AD converters within the control unit, thereby reducing the manufacturing cost.
A temperature detecting device according to this invention comprises: two heat-sensitive resistance elements arranged nearby an arbitrary heater and made even in characteristic; extraction-mode executing means for selectively executing a first voltage extraction mode for connecting the two heat-sensitive resistance elements in parallel between a power source and a ground through one of a pull-up resistance and a pull-down resistance and extracting a voltage of the two heat-sensitive resistance elements by a resistance divisional voltage, and a second voltage extraction mode for connecting any one of the two heat-sensitive resistance elements between the power source and the ground through one of a pull-up resistance and a pull-down resistance and extracting a voltage of the one heat-sensitive resistance element by a resistance divisional voltage; and determining means for determining whether or not the voltage extracted in the first extraction mode and the voltage extracted in the second extraction mode have a ratio matching a predetermined ratio, thereby determining a presence or absence of failure in the two heat-sensitive resistance elements.
Herein, the xe2x80x9cheat-sensitive resistance elementxe2x80x9d may be a non-linear output element such as a thermistor or a linear output element such as platinum ohmmeter. Meanwhile, although there are two kinds of element temperature characteristics, i.e. the type having a resistance value decreasing with increasing temperature (negative characteristic) and the type reverse to it (positive characteristic), any of these may be used.
According to this invention, provided that the pull-up resistance or pull-down resistance has a resistance value RP and the heat-sensitive resistance element has a resistance value RT, the voltage (V1) of the heat-sensitive resistance element in the case of the first voltage extraction mode is given by xe2x80x9cVCCxc3x97RT/(RP+RT)xe2x80x9d while the voltage (V2) of the heat-sensitive resistance element in the case of the second voltage extraction mode is given by xe2x80x9cVCCxc3x97RT/(2RP+RT)xe2x80x9d.
Accordingly, when the two heat-sensitive resistance elements are normally operating (no occurrence of disconnection or short circuit failure), the voltages respectively extracted in the first and second extraction modes have a ratio matching a ratio of the foregoing two equations (V1:V2), i.e. matches xe2x80x9c1/(RP+RT):1/(2RP+RT)xe2x80x9d. Thus, the two heat-sensitive resistance elements can be positively determined for a presence or absence of failure.
Meanwhile, in the case of separately providing a unit accommodating a heater and two heat-sensitive resistance elements and a unit accommodating a pull-up or pull-down resistance, extraction mode executing means and determining means and thereby connecting between these with a wire line, it is satisfactory to provide at least a first line connecting between the pull-up or pull-down resistance and the two heat-sensitive resistance elements, a second line connecting between one heat-sensitive resistance element and extraction mode executing means, and a third line connecting between the other heat-sensitive resistance element and a ground or power source. Thus, the number of lines can be reduced as compared to the prior art (see FIG. 14). Meanwhile, because of selective execution of first and second extraction modes, one AD converter can be shared between the modes, making it possible to reduce the number of AD converters and the number of pull-up or pull-down resistances.
A temperature detecting device according to this invention comprises: two heat-sensitive resistance elements arranged nearby an arbitrary heater and made even in characteristic; extraction-mode executing means for selectively executing a first voltage extraction mode for connecting the two heat-sensitive resistance elements in series between a power source and a ground through one of a pull-up resistance and a pull-down resistance and extracting respective voltages of the two heat-sensitive resistance elements by a resistance divisional voltage, and a second voltage extraction mode for connecting any one of the two heat-sensitive resistance elements between the power source and the ground through one of a pull-up resistance and a pull-down resistance and extracting a voltage of the one heat-sensitive resistance element by a resistance divisional voltage; and determining means for determining whether or not the two voltages extracted in the first extraction mode have a ratio matching a predetermined ratio, thereby determining a presence or absence of failure in the two heat-sensitive resistance elements.
According to this invention, provided that the pull-up resistance or pull-down resistance has a resistance value RP and the heat-sensitive resistance element has a resistance value RT, the voltage (Vo1) of one heat-sensitive resistance element in the case of the first voltage extraction mode is given by xe2x80x9cVCCxc3x972RT/(RP+2RT)xe2x80x9d while the voltage (Vo2) of the other heat-sensitive resistance element is by xe2x80x9cVCCxc3x97RT/(RP+2RT)xe2x80x9d.
Accordingly, when the two heat-sensitive resistance elements are normally operating (no occurrence of disconnection or short circuit failure), the voltages extracted in the first and extraction mode have a ratio matching a ratio of the foregoing two equations (Vo1:Vo2), i.e. xe2x80x9c1:2xe2x80x9d. Thus, the two heat-sensitive resistance elements can be positively determined for a presence or absence of failure.
Meanwhile, in the case of separately providing a unit accommodating a heater and two heat-sensitive resistance elements and a unit accommodating a pull-up or pull-down resistance, extraction mode executing means and determining means and thereby connecting between these with a wire line, it is satisfactory to provide at least a first line connecting between the pull-up or pull-down resistance and one of the heat-sensitive resistance elements and a second line connecting between the other end of the one heat-sensitive resistance element and extraction mode executing means. Thus, the number of lines can be reduced as compared to the prior art (see FIG. 14). Meanwhile, because of selective execution of first and second extraction modes, it is also possible to reduce the number of pull-up or pull-down resistances.
Meanwhile, according to a preferred embodiment of the invention, provided that the power source is VCC, one of the pull-up resistance and the pull-down resistance has a resistance value RP, and the two voltages extracted in the first extraction mode are respectively Vo1 and Vo2, a resistance value of the heat-sensitive resistance element of upon executing the second extraction mode by an equation xe2x80x9cRPxc3x97(Vo1xe2x88x92Vo2)/(VCCxe2x88x92Vo1)xe2x80x9d.
According to tis embodiment, by the equation xe2x80x9cRPxc3x97(Vo1xe2x88x92Vo2)/(VCCxe2x88x92Vo1)xe2x80x9d, it is possible to determine a correct resistance value of the heat-sensitive resistance element taken account of a ground potential difference. There is no need of providing a ground line between the two units, hence further making possible to reduce the number of lines.